Synthetic, plant and plasma antiproteinases inhibit the natural cytotoxicity (NK) of human peripheral blood lymphocytes to tumor cells. The role of proteinases in NK will be studied by in vitro substrate competition of NK and by identification of proteinases selectively active during NK. Class and substrate specificities of the proteinases will be determined by substrate competition of NK and by use of class-specific inhibitors. Identification and characterization of NK-specific proteinases will be made by radiolabeling of proteinases, and examination to see if they are unique, or in elevated concentration, during NK. Proteinases will be labeled with 3H DFP, or by 125 I after isolation of plasma antiproteinase-proteinase complexes formed during NK. Characterization will include determination of molecular weight and isoelectric focusing point, localization in the NK incubation medium or in subcellular components, and approximate quantitation by specific radioactivity and micro-Kjeldahl N analysis. The plasma alpha 1 antiproteinases that are most potent in inhibition of NK will be isolated and identified. Discrimination between effects on NK induction and/or mediation will be made by experiments to delay induction and to block NK of pre-induced cells. Time lapse cinematography will help determine whether the plasma antiproteinases inhibit the early interaction between monocytes and lymphocytes which results in induction of lymphocyte NK activity and attachment to tumor cells, or whether the plasma antiproteinases affect only the later events of tumor cell cytolysis. This study will contribute to understanding of the mechanism of cell-mediated cytotoxicity, and of the effects of the cellular environment upon cells cytotoxic to tumors.